The present invention relates to a display apparatus using electroluminescence elements.
Electroluminescence (EL) elements for use in a display apparatus include an inorganic EL element and an organic EL element. The inorganic EL element uses a thin film of an inorganic compound, like zinc selenide or zinc sulfide, as a fluorescent material, and the organic EL element uses an organic compound as a fluorescent material. Preferably, the organic EL element has the following features:
(1) A high external quantum efficiency. PA1 (2) Light is emitted on a low driving voltage. PA1 (3) Multifarious colors (green, red, blue, yellow, etc.) can be generated by selecting a proper fluorescent material. PA1 (4) The display is clear and no back light is needed. PA1 (5) There is no dependency on the viewing angle. PA1 (6) The organic EL element is thin and light. PA1 (7) A soft material like a plastic film can be used for the substrate.
Due to the aforementioned features, a display apparatus using such an organic EL element (hereinafter referred to as "organic EL display apparatus") is a desirable replacement for a CRT or liquid crystal display.
An organic EL display apparatus employs a dot matrix system which displays an image with dots arranged in a matrix form. The dot matrix system includes a simple matrix system or an active matrix system.
A conventional organic EL display apparatus 101 of the simple matrix system will now be discussed with reference to FIGS. 1 through 3.
As shown in FIG. 1, a plurality of anodes 103 are arranged, parallel to one another, on an insulator substrate 102, and a hole transporting layer 104 is provided on the insulator substrate 102 to cover the anodes 103. A light emitting layer 105 and an electron transporting layer 106 are formed on the hole transporting layer 104. A plurality of cathodes 107 are arranged, parallel to one another, on the electron transporting layer 106. The anodes 103 are placed perpendicular to the cathodes 107. The layers 104-106 are formed of an organic compound, and the layers 104-106, the anodes 103 and the cathodes 107 form an organic EL element 108. The insulator substrate 102 is preferably made of transparent glass, synthetic resin or the like. The anodes 103 are preferably formed of transparent electrodes of ITO (Indium Tin Oxide) or the like. The cathodes 107 are preferably formed of magnesium-indium alloy or the like.
In the organic EL element 108, holes coming from the anodes 103 are recombined with electrons coming from the cathodes 107 inside the light emitting layer 105, emitting light. The light is emitted outside via the anodes 103 and the transparent insulator substrate 102 as indicated by the arrow gamma (.gamma.) in FIG. 2.
The hole transporting layer 104 facilitates the injection of the holes from the anodes 103, and also blocks the electrons injected from the cathodes 107. The electron transporting layer 106 facilitates the injection of the electrons from the cathodes 107. The organic EL element 108 has a high external quantum efficiency, resulting in the display apparatus 101 having an improved luminous intensity.
FIG. 3 is a schematic plan view of the organic EL display apparatus 101, as viewed from the anodes 103. In FIG. 3, only the anodes 103 and the cathodes 107 are illustrated. In the organic EL element 108, defined at the individual intersections of anodes 103a to 103c and cathodes 107a to 107c are light emitting areas B1 to B9 which emit light, as discussed above. That is, the light emitting areas B1-B9, arranged in a matrix form, form pixels of the organic EL display apparatus 101.
In a simple matrix system, the positive terminal of a driving power supply is connected to the anodes 103, and the negative terminal of the driving power supply is connected to the cathodes 107. In this manner, the anodes 103 and the cathodes 107 are energized.
In order for the light emitting area B2 at the intersection of the anode 103b and the cathode 107a to emit light, for example, the positive terminal is connected to the anode 103b and the negative terminal is connected to the cathode 107a, so that power is supplied through the terminals. As a result, a forward current flows, as indicated by the arrow alpha (.alpha.)When energizing the light emitting area B2, a leak current may flow, as indicated by the arrow beta (.beta.). The leak current energizes not only the light emitting area B2, but also the light emitting areas B1, B3 and B5 near the light emitting area B2. As a result, the light emitting areas B1, B3 and B5 emit light. This phenomenon is called optical crosstalk caused by the leak current characteristic of the EL element 108.
The light produced by the light emitting layer 105 can be scattered. As indicated by the arrow delta (.delta.) in FIG. 2, the light from the light emitting layer 105 is reflected at the electron transporting layer 106 and can be discharged outside from an unnecessary location. Further, the light from the light emitting layer 105 can be discharged outside from an unnecessary location without passing the anodes 103 as indicated by the arrow epsilon (.epsilon.) in FIG. 2. As indicated by the arrow eta (.eta.) in FIG. 2, the light from the light emitting layer 105 is optically guided by the optical waveguide effect resulting from the difference in refractive index between the light emitting layer 105 and the hole transporting layer 104, and can be discharged outside from an unnecessary place. This light scattering causes light emission at a location other than the desired pixel in the organic EL display apparatus 101. This phenomenon is called the generation of optical crosstalk caused by light scattering that has originated from the structure of the EL element.
The optical crosstalk due to the leak current and the structure of the EL element 108 deteriorates the contrast of images displayed by the organic EL display apparatus 101, preventing the acquisition of high-definition images. Particularly, a full-color organic EL display apparatus using EL elements causes color "smearing" and does not provide clear images.
The simple matrix system directly drives organic EL elements 108 of a matrix of pixels, arranged on a display panel, in synchronism with a scan signal using an external driving unit. Each pixel on the display panel has only an organic EL element. As the number of scan lines of a display apparatus increases, therefore, the driving time (duty) assigned to each pixel decreases. This reduces the contrast as well as the luminance intensity of the display screen.
Accordingly, it is an object of the present invention to provide a display apparatus, using electroluminescence elements, capable of displaying clearimages.